The environment surrounding all of us consists of numerous pathogens: agents (usually microorganisms) that cause disease(s) in their hosts. A host is the organism that is invaded and often harmed by a pathogen. Pathogens, which include bacteria, protists, fungi, and other infectious organisms, can be found in food and water, on surfaces, and in the air. Concern over pathogens is one of the main reasons that we wash our hands after going to the bathroom or touching raw meat.
Mammalian immune systems evolved for protection from such pathogens. They are composed of an extremely-diverse array of specialized cells and soluble molecules that coordinate a rapid, flexible defense system capable of providing protection from a majority of these disease agents (Figure 1). Central to this goal, the immune system must be capable of recognizing "self" from "other" so that when it destroys cells, it destroys pathogen cells and not host cells.
The immune response that defends against pathogens can be classified as either innate or active. The innate immune response is present in its final state from birth and attempts to defend against all pathogens. Conversely, the adaptive immune response stores information about past infections and mounts pathogen-specific defenses. It expands over time, gaining more information about past targets so that it can respond quickly to future pathogens. The adaptive immune response functions throughout the body to combat specific pathogens that it has encountered before (a process known as reactivation). However, we are born with only innate immunity, developing our adaptive immune response after birth.
Components of both immune systems constantly search the body for signs of pathogens. When pathogens are found, immune factors are mobilized to the site of an infection. The immune factors identify the nature of the pathogen, strengthen the corresponding cells and molecules to combat it efficiently, and then halt the immune response after the infection is cleared to avoid unnecessary host cell damage. Features of the immune system (e.g., pathogen identification, specific response, amplification, retreat, and remembrance) are essential for survival against pathogens.